The Tarn (Occitan: Tarn, Latin: Tarnis, possibly meaning 'rapid' or 'walled in') is a 381-kilometre (237 mi) long river in southern France (régions Languedoc-Roussillon and Midi-Pyrénées), right tributary of the Garonne.

The Tarn runs in a roughly westerly direction, from its source at an altitude of 1,550 m on Mont Lozère in the Cévennes mountains (part of the Massif Central), through the deep gorges and canyons of the Gorges du Tarn (that cuts through the Causse du Larzac), to Moissac in Tarn-et-Garonne, where it joins the Garonne 4 km (2.5 mi) downstream from the centre of town.

Its basin covers approximately 12,000 km², and it has a mean flow of approximately 140 m³ per second.

The Millau Viaduct spans the valley of the River Tarn near Millau, and is now one of the area's most popular attractions.